This invention relates to a method for improving the intermodulation distortion performance of a single sideband system by predistortion.
The traveling wave tube amplifiers normally used in microwave single sideband amplitude modulated transmitters have significant nonlinearities which introduce intermodulation distortion. In order for the transmitter exhibit greater linearity than the traveling wave tube amplifier, the transmitter can include a "predistortion" circuit which introduces a nonlinearity that is the inverse of that in the traveling wave tube amplifier.
Conventionally, predistortion is not used in receivers, because they can be made linear over a useful power range without it. If a receiver receives more power than it can handle linearly, attenuators can be used to reduce the power. Unfortunately, this reduces the fade margin as well.
In aligning a transmitter in the laboratory or factory using predistortion, radio frequency power from the transmitter is coupled to an associated receiver at a power level that is reduced sufficiently that the receiver makes negligible contribution to intermodulation distortion in the signal path. The intermediate frequency output of the receiver operating in this condition is monitored for intermodulation distortion, while the predistortion circuit nonlinearity is adjusted.
Predistortion alignment of the transmitter in the field transmitting to a distant site is more difficult. An example arrangement for performing such an alignment is shown in FIG. 1. A coupler 10 samples the radio frequency output from the transmitter 12 under test. The level of the signal from coupler 10 is reduced by attenuator 14 to a sufficiently low level that test downconverter 16 will not introduce significant intermodulation distortion. A reject filter 18 removes intermodulation distortion-producing test tones before monitoring of the downconverted intermediate frequency signal by a test set 20. Precautions must be taken to ensure that no frequency errors occur during the translation by downconverter 16. As shown in FIG. 1, local oscillators 22 and 23 of the transmitter 12 and downconverter 16, respectively, are locked to the output of a common reference frequency source 26. Another method employed is to sample the transmitter local oscillator to drive the test downconverter.